The wing pattern of the Milbert's Tortoiseshell cannot be confused with that of any other butterfly. Males and females are almost identical, but male forelegs are hairier than female forelegs.
Eggs are green and conical, with a flattened base and a rounded top, and 8-9 vertical ribs. First instar larvae are yellow green, with rows of black tubercles. Each tubercle has a long black barbed hair. Mature larvae are black on the top half, and yellow green on the bottom half. They are thickly sprinkled with small yellow white or white dots and fine whitish hairs, producing an overall greyish appearance. The body is covered with branching black spines. There is a greenish yellow line down each side, with another line of brighter orange yellow dashes above it. The spines coming from the greenish yellow line are also greenish yellow. The underside of the body is greyish green. Pupae are generally brown with numerous brilliant metallic gold points, but are highly variable. Pupae that are parasitized by ichneumonid wasps have dull red abdomens, and the thorax and head are polished gold tinged with green (Saunders 1869c; Gosse 1883; Edwards 1885c).
The nominate subspecies, A.m. milberti (Godart, ) (TL: Philadelphia, PA) occurs throughout BC.
The name Aglais is from the Greek aglaos (beautiful) or aglaia (beauty) (Emmet 1991). The common name "tortoiseshells" refers to the upperside of the wings of the butterflies resembling the mottled yellowish brown "tortoiseshell" of some sea turtles (Comstock and Comstock 1915), which is sometimes used for making combs. The name is shared with the genera Nymphalis and Roddia because the species in all three genera were originally placed in the genus Aglais.
The genus Aglais is defined on the basis of differences in genitalia compared with Nymphalis, such as an elongate penis and a long saccus (Layberry et al. 1998). In addition, the mitochondrial DNA comparisons of Soren Nylin (pers. comm.) indicate that Aglais is sufficiently distant from Nymphalis to be treated as a separate genus. There are four species worldwide.
There is only one species in the genus native to North America, the Milbert's Tortoiseshell. A related European species, the Small Tortoiseshell (Aglais urticae), has recently been accidentally introduced to North America near New York and may eventually become more widely established. A previous introduction in 1875 near Waterton, MA (Anonymous 1875) apparently did not become established.
Eggs are laid in masses on stinging nettle leaves, and the larvae form an aggregation for feeding. They feed in aggregations and skeletonize the nettle leaves through the first three instars. Some webbing is spun, apparently to provide a secure foothold. Fourth instar larvae begin dispersing and cut, fold, and roll leaves around themselves in the same way as the other nettle-feeders: Satyr Anglewings, West Coast Ladies, and Red Admirals. More than one larva may inhabit a single folded leaf, and larvae of more than one nettle-feeding butterfly species may share a single leaf. Larvae in the fifth and final instar disperse and feed in the open, without rolling leaves (Edwards 1885c; CSG).
Milbert's Tortoiseshells are univoltine in BC but are bivoltine or trivoltine in eastern Canada and Colorado (Layberry et al. 1998; Brown et al. 1957). Adults emerging in July may aestivate until fall and then hibernate for the winter (Guppy 1955). They overwinter successfully near Vernon (Venables 1913), and it is likely that they do so throughout BC. They may also hibernate as both larvae and pupae in Colorado (Ferris and Brown 1981). Shapiro (1974a, 1979) suggests that in California milberti adults hibernate at low elevations on the coast and breed there in the spring, with the resulting offspring migrating to higher elevations. The upward movement appears to occur in the Southern Interior of BC as well, but with no indication of a fall movement back to low elevations. Population densities are usually too low to permit monitoring of elevational movements. Larvae suffer an extremely high rate of parasitism (Edwards 1885c). All of the about 50 Milbert's Tortoiseshell larvae collected on southern Vancouver Island (Guppy 1951), and all but one of about 100 larvae collected at Botanie Valley near Lytton (CSG), were parasitized by tachinid flies.
Stinging nettle (Urtica dioica) is the larval foodplant in BC (Dyar 1904b; Harvey 1908; Jones 1933; Guppy 1951; Atkinson 1978; CSG; FIS) and in most other places. In Colorado, willows and wild sunflowers (Helianthella) are reportedly used in addition to stinging nettle (Brown et al. 1957), which seems improbable.
Milbert's Tortoiseshells are found throughout BC at all elevations. In August they are frequently numerous in mountain meadows, to which they have flown for nectar from the low-elevation areas where breeding occurs. Breeding occurs through northern BC, with numerous larvae found near the Tatshenshini River in the St. Elias Mountains of extreme northwestern BC (CSG).
Milbert's Tortoiseshells occur from southeastern AK east across the southern half of CAN to NF. In the west they occur south to CA and NM almost to the Mexican border, and east of the Rockies south to the central USA.
Recommended citation: Author, Date. Page title. In Klinkenberg, Brian. (Editor) 2019. E-Fauna BC:
Electronic Atlas of the Fauna of British Columbia [efauna.bc.ca]. Lab
for Advanced Spatial Analysis, Department of Geography, University of British
Columbia, Vancouver. [Accessed:
2021-03-08 2:40:29 PM]
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